1. Field of the Invention
The present invention is related to the field of holography, and constitutes an apparatus for projecting high quality images during the process of recording holographic stereograms.
2. Description of the Related Art
In the field of holographic imaging, a certain type of synthetic hologram, generally called a stereogram, is constructed by the sequential recording of a consecutive series of two dimensional perspective views. One method of capture for an appropriate sequence of images is to pass a 35 mm movie camera along a straight course, in front of a real scene or object and record the target object from various viewpoints. Of greater current interest is a second method of sequence generation which involves the application of computer graphics rendering techniques to three-dimensional databases. Whatever the method of capture, the sequence of images are subsequently recorded in a holographic medium, in a spatially multiplexed one-dimensional (horizontal parallax only) or two-dimensional (horizontal and vertical parallax) array. The holographic recording apparatus is shown in FIG. 6.
The holographic recording apparatus comprises (a) a holographic recording medium(51) standing immediately behind an opaque barrier(52), (b) the opaque barrier(52) containing a narrow slit(53) whose size is generally between 1 to 3 mm, (c) a diffusing screen(54) standing on the side of the opaque barrier(52) opposite to the holographic recording medium(51), the diffusing screen(54) being spatially separated from the opaque barrier(52) by a distance determined by the intended display parameters of the stereogram under construction, and (d) projection apparatus(55) sitting at some distance beyond the diffusing screen(54) on the side opposite to the opaque barrier(52) and the recording medium(51).
The recording method involves using a laser beam with the projection apparatus(55) to rear project an image onto the diffusing screen(54). The laser light from this projected image diffuses from every point on the screen(54) through the narrow slit(53) in the opaque barrier(52) and falls on the recording medium(51). A second laser beam(56), coherent to the projection beam, is simultaneously passed through the slit(53), where it interferes with the diffused image light, and this interference pattern is recorded as a hologram. The overall recording process involves moving the slit(53) after each recording by a distance equal to the slit width, revealing a neighboring section of the recording medium(51) and recording therein a projected image of the scene from an adjacent perspective view. The stereogram is built up by this step and repeat method of holographically recording adjacent perspective views in neighboring portions of the recording medium(51).
In the current state of the art images for holographic stereograms that are formed using computer graphic techniques are projected by one of two methods. In the first method, computer generated images are recorded on photographic film. After processing, the film is used as the projection medium. In the second method, a spatial light modulator, usually a liquid crystal television panel, is used as the projection medium. A representation of projection is shown in FIG. 7. A plane wave(57), formed of collimated laser light, transits an image bearing projection medium(58) which serves as the integral element of the projection apparatus(55) from FIG. 6. The light field, modulated by the image, travels to the diffusing screen(54), where the image becomes visible by the action of the diffusing screen(54).
The functional tradeoff between the two projection technologies is that while photographic film provides a high quality continuous image to the diffusing screen(54) and, thus, subsequently to the recording medium(51) via the slit(53), the fact that the film must be developed before it can be used as the projection medium(58) prevents this method from being completely automated. On the other hand, the spatial light modulator method allows complete automation of the stereogram recording process, but it provides an image of significantly reduced quality, most significantly containing a gridlike artifact, a consequence of the electronic control lines which matricize the projection panel.
FIG. 8 displays the case of a Liquid Crystal panel(59) performing the function of the projection medium(58) of FIG. 7. When the plane wave(57) encounters the LCD(59), portions of the plane wave are blocked by opaque cells(60), while other portions of the wave pass through transparent cells. When these portions of the wave that pass through the LCD fall incident upon the diffusing screen(54), they illuminate a portion(61) of the diffusing screen(54) proportional to the size of the cell, or pixel, of the LCD through which they passed. Because adjacent cells on the LC panel(59) are spatially separated to accommodate the thin- film-technology (TFT) control electronics(62), the portions(61) of the diffusing screen(54) illuminated by adjacent pixels will not abut, but will also have a spatial separation(63). This pixel separation(63) on the diffusing screen(54) is the source of the above mentioned gridlike artifact which limits the quality of the images produced by LCD projection devices.
The origin of this gridlike artifact can be further understood through FIG. 9. The degree to which an individual pixel(64) is transparent to light is managed by the action of the control electronics(65). These control electronics(65) are often protected by a shield matrix(66) to prevent overheating. The open area in the shield matrix forms an entrance aperture(67) to the pixels, and the open area between the control electronics(65) matrix forms an exit aperture(68). Even if every pixel in the LCD is set to be totally transparent, only that portion of the propagating wavefront(69) that will pass through both apertures(67,68) will continue to propagate. The shadow of the-shield matrix(66) and control electronics matrix(65) will form the gridlike artifact in the projected image.